On the automorphism groups of the Z₂Z₄-linear Hadamard codes and their classification

Publicació amb motiu del 4th International Castle Meeting (Pamela Castle, Portugal). Sep. 15-18 2014It is known that there are exactly ⌊(t−1)/2⌋ and ⌊t/2⌋ nonequivalent Z₂Z₄-linear Hadamard codes of length 2ᵗ , with α = 0 and α≠0, respectively, for all t ≥ 3. In this paper, it is shown that each Z₂Z₄-linear Hadamard code with α = 0 is equivalent to a Z₂Z₄-linear Hadamard code with α ≠ 0, so there are only ⌊t/2⌋ nonequivalent Z₂Z₄-linear Hadamard codes of length 2ᵗ. Moreover, the orders of the permutation automorphism groups of the Z₂Z₄-linear Hadamard codes are given

Classification of the Z₂ Z₄-linear Hadamard codes and their automorphism groups

Combinatorics, Coding and Security Group (CCSG)A Z₂ Z₄-linear Hadamard code of length α + 2β = 2t is a binary Hadamard code, which is the Gray map image of a Z₂ Z₄-additive code with α binary coordinates and β quaternary coordinates. It is known that there are exactly ⌊t-1/2⌋ and ⌊t/2⌋ nonequivalent Z₂ Z₄-linear Hadamard codes of length 2t, with α = 0 and α ≠ 0, respectively, for all t ≥ 3. In this paper, it is shown that each Z₂ Z₄-linear Hadamard code with α = 0 is equivalent to a Z₂ Z₄-linear Hadamard code with α ≠ 0, so there are only ⌊t/2⌋ nonequivalent Z₂ Z₄-linear Hadamard codes of length 2t. Moreover, the order of the monomial automorphism group for the Z2Z4-additive Hadamard codes and the permutation automorphism group of the corresponding Z₂ Z₄-linear Hadamard codes are given